Integrated circuit or microcircuit chips are generally mounted on lead frames for coupling to external circuitry. Such lead frames are stamped in long strips or lengths of metal ribbon. A fragmentary portion of such a lead frame strip 6 showing two successive lead frames 5 is illustrated in FIG. 1. Each lead frame 5 includes a paddle 7 to which the microcircuit chip is secured by epoxy resin, and a plurality of lead frame fingers or prongs 8 which extend from a "dam" 9 toward the paddle to receive the lead wires from the chip. After lead frames have been stamped in long lengths of metal ribbon, the ribbon is mounted on large reels for shipping to the customer.
The lead frames are generally stamped from copper alloy, red brass, stainless steel, and similar metals. According to the conventional methods of lead wire bonding, the paddle and lead frame fingers must be plated with gold or silver in order to achieve good quality welds. In order to prepare the lead frames, the reels of stamped ribbon are shipped to another location for plating and stamping or cutting into shorter strips. The lead frames are thereafter shipped again to the microcircuit chip manufacturer for packaging the chips. Because of the extensive shipping and handling required the lead frame ribbons are frequently bent or damaged particularly during the shipping and handling of the large reels.
Bonding of lead wires between a microcircuit chip and the lead frame on which the chip is mounted is generally accomplished by "ball/wedge" bonding. According to this technique a lead wire or bonding wire 11 is held in a capillary tool 12 with the lead wire 11 projecting beyond the end of the tool as shown in FIG. 1A. The capillary tool 12 forms part of a bonding machine in which the tool is appropriately mounted and positioned over the metallized die pad 14 of an integrated circuit chip or substrate 17, mounted on paddle 7. As shown in FIG. 1A a ball 15 of metal has been formed at the end of the lead wire or bonding wire 11 by melting for example with a hydrogen gas flame torch or by electric arc discharge.
After rehardening of the metal ball 15 of the ball end of the wire is brought into intimate contact with the metallized die pad 14 as shown in FIG. 1A. A bond is formed typically by thermocompression bonding applying a specified force and temperature for a specified period of time. Metallic welding and diffusion combine to form this basic bond. Alternatively, ultrasonic bonding or other form of welding may be used.
The capillary tool 12 and substrate 17 are then moved relative to each other for bonding of the wire to lead frame finger 8. At this location a wedge bond between the lead wire 11 and finger 8 is generally formed and the lead wire or bonding wire 11 is severed below the bonding tool at the weld. The wedge bond is formed by thermocompression or ultrasonic bonding with the edge of the capillary tool bearing against the wire and lead frame finger. In this manner a lead wire connection is established between the metallized substrate of a chip and the lead frame for coupling to external circuitry. Further background on bonding of lead wires can be found in the article "Evaluating Wire Bond Quality" by Steven W. Hinch and Donald R. Cropper in the February 1981 issue of Semiconductor International, and in copending U.S. patent application Ser. No. 262,595 filed May 11, 1981 entitled "Bonding Wire Ball Forming Method and Apparatus", assigned to the common assignee of the present invention and now U.S. Pat. No. 4,390,771.
The conventional approach to securing lead wire or bonding wire between the microcircuit chip and lead frame fingers is limited to methods in which the chip and lead frame finger are maintained at the same relatively low level temperature, a temperature below the level at which damage might occur to the chip. Thus, thermocompression and ultrasonic welding or bonding methods are used at temperatures typically no greater than for example 200.degree. C. The ball bond must be formed first because it requires the use of the free end of the wire. Since this is the preferred bond at the chip, the ball bond to the chip is formed first. The capillary tool then bears against the lead frame finger to effect the second bond. In the conventional approach the lead frame finger is therefore electrically and thermally coupled to the chip during the second bond. Heating of the second bond by application of electrical energy is therefore severely limited because of the damage which might occur at the chip due to excessive voltage, current or temperature.
At these lower bonding energies, gold or silver plating of the bonding surfaces, i.e. the lead frame fingers, is necessary in order to achieve secure welds with good electrical contact. Furthermore, gold or silver lead wire and bonding wire is also generally used for the same reasons.
Copending U.S. patent application Ser. No. 262,595 referred to above describes a new method and apparatus for ball bonding to the microcircuit chip using lead wires of copper, aluminum and other reactive metals in addition to gold and silver. The present invention carries this approach further to achieve high quality welds at the lead frame fingers for lead wires of copper etc. and furthermore without the need for gold or silver plating of the lead frame fingers.